Abstract We seek to understand the mechanisms underlying ovulation and luteal formation in the human by determining the function of key ovulatory mediators, progesterone (P4) and progesterone receptor (PGR) in human periovulatory follicles. The understanding of P4/PGR-mediated cellular events underlying ovulation and luteal formation has strong translational potential because P4 and its antagonists have been used as contraceptives and the modulation of P4 levels is one of most common practices to aid or prevent female fertility. The increase in P4 production and PGR expression in periovulatory follicles after the LH surge has been found to be crucial for successful ovulation in many species. For instance, Pgr knockout mice fail to ovulate. The inhibition of P4 synthesis and PGR action blocked ovulation and/or luteal function in studies using rodents, sheep, and non-human primates. However, there are very few reports of PGR expression in the human ovary and virtually no experimental data on the mechanism(s) of P4/PGR's action in human periovulatory follicles. In the present proposal, we hypothesize that PGR induced by the LH surge in periovulatory follicles plays a crucial role in ovulation and/or luteal formation by regulating the expression of key ovulatory and/or luteal genes in women. To test this hypothesis, we will utilize a unique in vivo model that allows the collection of timed human periovulatory follicles from normally cycling women and a primary human granulosa cell culture model in which the LH-induced increase in the expression of PGR and other key ovulatory genes in vivo can be mimicked in vitro. We proposed first to unveil potential downstream target genes of P4/PGR using a primary human granulosa cell culture model (Specific Aim #1). Secondly, we will determine the expression pattern of PGR-downstream target genes in preovulatory follicles obtained before the LH surge, periovulatory follicles obtained at defined hours after hCG administration and post-ovulatory follicles from normally cycling women (Specific Aim #2). This study will be the first to document the comprehensive expression pattern of key and novel ovulatory genes in human periovulatory follicles during the ovulatory period in vivo and identify P4/PGR-target genes to delineate the mechanisms by which P4/PGR mediates the ovulatory process in humans.